What is the right polygon count?
Is there a max tris count number that I should not exceed?


Nothing you need to worry about it. I'm sure nobody managed to get that many vertices that hit that limit and can still use Blender. Remember you have limited memory ;).


  Is there a default tris count number for small(ornaments)/medium(furnitures)/big(houses/terrains) that I should be aware of? 
  No
  
  ... do I keep checking till the fps drops and try to dissolve meshes afterwards?


It is the purpose of profiling to identify bottlenecks. It depends how much effort you want to investigate in it.


  Objects in different layers or stand-by scenes that might appear briefly or not appear at all in the game affect the fps?


Not loaded scenes just cost disk space.

Inactive objects (residing in hidden layers) just cost memory and load time, but no processing time while the game is running.

Added Objects need additional processing time to get added (which just a little). After that they are treated as normal objects.


  Objects with dynamic physics or objects with animations or characters with armatures or in general objects that interact with one way or another, do they need lower poly count than a similar one that would not act like the above and let's say would remain static and act as a decorative level asset?


Every detail causes costs in one way or the other. If you do not need a detail it is more efficient to skip that detail.

Example: A tree can have a lot of polygonal details for rendering but much less regarding physics. Other details are better presented in a texture rather then lots of polygons.

In general it is better to care such things when you have performance problems. Otherwise you spend time on aspects that might not be relevant to your situation.

Surely this is different when working on large projects, with several team members, strict budget and deadlines. Even than you will hit performance issues.